One conventional fire and overheat sensor is known as a "thermal wire". This system senses a fire or overheat condition by thermal conduction from ambient to the center of a 1/16 inch diameter stainless steel tube. The sensing element may be a hydride which generates a gas as the temperature increases, the generated gas being sensed by a pressure switch. Alternatively the sensing element may be a salt which melts as temperature increases thus causing a change in an electrical resistivity vs. temperature characteristic of the sensing element.
Another conventional fire and overheat sensor employs a far-infrared optical detector to detect radiometric heat in combination with a two spectrum, far-near infrared fire detector.
However, for many high ambient temperature applications, such as jet aircraft engine nacelles, this latter type of system may not be usable in that the system typically has a maximum ambient temperature limitation of approximately 400.degree. F. This maximum ambient temperature limitation is due in large part to the maximum temperature limits of the sensor electronics.
The thermal wire type of system, which typically has a higher ambient temperature limitation, is suitable for use in an engine nacelle. However, this type of system has a relatively slow response time. This type of system furthermore may not detect as many as 40% of confirmed fires while exhibiting up to a 60% false alarm rate.
In U.S. Pat. Nos. 4,701,624, 4,691,196, 4,665,390 and 4,639,598, all of which are assigned to the assignee of this invention, there are described fire sensor systems which have overcome the problems inherent in the aforementioned thermal wire type of system. These systems accurately and rapidly detect the occurrence of a fire while also eliminating false alarms. However, in that these systems employ wavelengths of less than two microns they generally cannot also simultaneously be employed for detecting overheat conditions in a radiometric fashion as described in U.S. Pat. No. 4,647,776, which is assigned to the assignee of this patent application.
It is thus an object of the invention to provide both a flame and heat sensing system which employs wavelengths of less than two microns for flame detection while simultaneously detecting an overheat condition.
It is a further object of the invention to provide a flame and heat sensing system which employs wavelengths of less than two microns for flame detection while simultaneously detecting an overheat condition such that an actual flame condition is not required to generate an alarm condition.
It is also an object of the invention to provide a capability to upgrade a fiber optic fire sensor system with a capability to detect an overheat condition.
It is one further object of the invention to provide a flame and an overheat detection system for use in an environment having a high ambient temperature, such as an aircraft engine nacelle, and which further eliminates the undetected fire and false alarm deficiencies of conventional systems, such as thermal wire systems.
It is a further object of the invention to provide a fiber optic flame detection system with an overheat condition detection capability by employing a temperature dependent fluorescence characteristic of a material which is disposed at a far end of the fiber, the material being pulsed with optical radiation at a first wavelength and a fluorescent response of the material being determined at a second wavelength.
It is a further object of the invention to provide an overheat detection capability with a minimum of additional components at the distal end of a fiber and with few or no additional components in the fire sensor electronics, beyond those components that would be required for a built in test of the fiber and electronics.
It is also an object of the invention to provide signal processing circuitry such that a fire sensing function and an overheat sensing function do not interfere with one another even though these two functions may share the same fiber, detectors and circuitry.